docs: def step3(self):
          """Last authentication step."""
          self._check_initialized()
          # TODO: verify: self.client_session_key same as self.session.key_b64()?
          session_key = binascii.unhexlify(self.client_session_key)
  
          aes_key = hash_sha512('Pair-Setup-AES-Key', session_key)[0:16]
          tmp = bytearray(hash_sha512('Pair-Setup-AES-IV', session_key)[0:16])
          tmp[-1] = tmp[-1] + 1  # Last byte must be increased by 1
          aes_iv = bytes(tmp)
          log_binary(_LOGGER, 'Pair-Setup-AES', Key=aes_key, IV=aes_iv)
  
          epk, tag = aes_encrypt(modes.GCM, aes_key, aes_iv, self._auth_public)
          log_binary(_LOGGER, 'Pair-Setup EPK+Tag', EPK=epk, Tag=tag)
  
          return epk, tag